As present day electronic components become smaller, the density (number) of components on circuit boards has generally increased. The increased density of components makes manual inspection of circuit boards to detect missing and misplaced components extremely difficult. For this reason, many electronics manufacturers are turning to machine vision systems to accomplish the task of circuit board inspection.
Allowed U.S. patent application, Ser. No. 111,954, now U.S. Pat. No. 4,811,410, filed in the name of I. Amir et al. on Oct. 23, 1987, and assigned to AT&T (incorporated by reference herein), discloses a system for inspecting a circuit board to detect missing and misaligned components. The Amir et al. inspection apparatus incorporates a linescan camera which contains a plurality of charge coupled devices (ccd's), each serving to capture the image of a small area or picture element (pixel). The ccd's in the linescan camera are arranged in a 1.times.N (N being the number of ccd's) linear array such that the ccd's collectively capture the image of a thin strip of surface area running across the circuit board undergoing inspection.
The ability of the linescan camera to capture the image of a long, but narrow, strip of surface area affords the camera the advantage of a wide field of view, as compared to other types of television cameras. With its wide field of view, the linescan camera can capture the image of a circuit board in a single pass. In contrast, with a conventional television camera, the field of view usually must be restricted to a very small area on the circuit board in order to obtain sufficient resolution for high inspection accuracy. Hence, it is usually necessary to "step" or displace such conventional cameras across the x and y axis of the circuit board in order for the camera to capture the entire image of the board.
One of the difficult tasks in implementing an inspection system using a linescan vision camera is to provide homogeneous illumination. In particular, achieving homogeneous side lighting of the circuit board, where light is directed at the surface at a grazing angle, in order to inspect for certain types of components, has heretofore proven to be very difficult. Simply directing light towards the board from each of its sides at a grazing angle usually does not provide sufficient intensity for good image resolution by the camera along its optical axis. The intensity of the light decreases proportionally to the square of the distance from the light source, so the light radiated from the edges of the circuit board will usually suffer too great a loss in intensity before reaching the center of the board.
Therefore, there is a need for a technique for homogeneously illuminating a substrate with side light.